Over the last hundred years, the human skull has gotten rounder, broader at the jaw, and more distinctly different between men and women.
The comparison of the Japanese skulls from the early 1900s with those of people who died in the 2020s found measurable changes in cranial shape across a single century. Present-day skulls are wider from side to side, shorter from front to back, have a larger bony projection behind the ear, and a wider upper jaw, researchers report in the American Journal of Biological Anthropology.
“The present study is the first to demonstrate detailed 3D secular changes of the cranium over the past century in modern Japanese,” the team, led by Shiori Usui at the National Research Institute of Police Science in Japan, writes in the paper.
To make the comparison, the Usui and her colleagues created detailed digital models of 56 historical skulls housed at Kyoto University Museum, belonging to individuals who lived between roughly 1900 and 1920, alongside 56 postmortem CT scans from people who died between 2022 and 2024. Each skull was mapped using 161 reference points, and software was used to compare overall shapes across the two groups.
The most prominent change was a shift toward a rounder, wider skull relative to its length — a process called brachycephalization. The mastoid process, a cone-shaped protrusion behind the ear that anchors neck muscles, grew larger and more prominent. The nasal bone became more protruding but narrower, the forehead slightly more concave, and the upper jaw relatively wider, even as the cheekbone narrowed.
A century ago, sex differences in skull shape were relatively subtle, with the two groups overlapping considerably. Today, the gap has widened. Male skulls now show a more pronounced brow ridge, a more steeply sloping forehead, a more projecting face, and a larger mastoid process compared with female skulls. “Sexual dimorphism in Japanese cranial morphology was relatively small about 100 years ago but has become more pronounced over the past century,” the researchers write.
The underlying causes remain uncertain. The authors point to well-documented trends, including increases in overall body size and improvements in nutrition. “It is also possible that dietary changes, such as greater consumption of soft foods and the resulting reduction in chewing load, have contributed to modifications of the facial skeleton and mandible,” they note.
Growing bones respond to the mechanical forces placed on them, and softer diets mean less stress on the jaw and face during childhood. Because the study focused entirely on Japanese individuals, whether these patterns hold across other populations remains to be tested.
Journal Reference: American Journal of Biological Anthropology. DOI: 10.1002/ajpa.70235
